(1) Field of the Invention
This invention relates to a process for producing chloroprene rubber. More particularly, it relates to a process for producing chloroprene rubber characterized in that it exhibits an enhanced whiteness, i.e., a reduced yellowness and, when it is used as an adhesive, an appropriate tackiness of the adhesive applied on an adherend can be kept over a long period.
(2) Description of the Related Art
Chloroprene rubber is highly crystalline and exhibits a large cohesive force, and therefore, it is widely used as adhesives. Among the chloroprene rubber adhesives, a solution-grafted chloroprene rubber adhesive is known as useful for adhering plasticized vinyl chloride resin adherends, which adhesive is prepared by incorporating a monomer such as, for example, methyl methacrylate and a radical polymerization initiator such as benzoyl peroxide in a solution of chloroprene rubber in an organic solvent such as toluene or methyl ethyl ketone, whereby a graft polymerization is caused. The solution-grafted chloroprene rubber exhibits an enhanced adhesion and therfore has heretofore been used popularly for adhering synthetic leather substitutes such as in synthetic leather shoes.
Processes for producing chloroprene rubber are known from of old. One typical example of the production processes is described in Encyclopedia of Polymer Science and Engineering, 2nd ed., vol. 3, Wiley-Interscience, New York, 1985, p447-449. In this production process, first, an emulsion polymerization is conducted by using a soap such as an alkali metal salt of a disproportioned rosin acid as an emulsifier, and an emulsion stabilizer such as, for example, an alkali metal salt of a condensate of naphthalenesulfonic acid with formaldehyde; then unreacted monomer is removed by steam flashing, i.e., steam distillation under conditions such as, for example, a temperature of 50.degree. C. and an absolute pressure of 100 Torr; and finally the polymer latex is subjected to freeze-coagulation or salting-out to give chloroprene rubber.
In the final step, the polymer latex as obtained by the emulsion polymerization and having a pH value of 12 to 13 is acidified to a pH value of 5 to 7 by adding dilute acetic acid. By the acidification, the alkali metal salt of the disproportioned rosin acid used as the emulsifier changes to an acid form, and thus, loses an emulsifying function. Therefore, unless an emulsion stabilizer, i.e., a surface active agent capable of imparting a stability to the polymer emulsion latex, is made present therein, the latex is undesirably coagulated and thus it becomes impossible that the latex is subjected to freeze-coagulation or salting-out. As an example of the emulsion stabilizer, there can be mentioned an alkali metal salt of a condensate of naphthalenesulfonic acid with formaldehyde. By the incorporation of the emulsion stabilizer, the polymer latex is stabilized and can be subjected to freeze-coagulation or salting-out on a rotating cooling drum. The freeze-coagulated polymer latex is then melted, washed and mechanically compressed into a film form, which is then dried on a belt by hot air in a dryer.
Chloroprene rubber produced by the above-mentioned process has a shade of white tinged with yellowish brown. The L value, a value and b value of the chloroprene rubber as measured by Lab reflection are in the ranges of 65 to 75, -2.0 to -3.0 and 10.0 to 16.0, respectively. An adhesive made from the chloroprene rubber is characterized in that the yellowness index as determined according to the light absorption is in the range of 0.4 to 0.6 and thus the adhesive does not satisfactorily meet the requirements for synthetic shoes that the adhesive must be transparent and colorless.
To improve the conventional chloroprene rubber, a proposal has been made in Japanese Unexamined Patent Publication No. 57-34844 wherein a combination of an alkali metal salt of a disproportioned rosin acid with a specific additional surface active agent is used as an emulsifier. As specific examples of the additional surface active agent, there can be mentioned alkali matal salts of a C.sub.12-15 alkylsulfonic acid, alkali metal salts of a 1,3-bis-(2-ethyl-hexyl)glycerol-ether sulfuric acid ester, alkali metal salts of .alpha.-hydroxy-octadecanesulfonic acid, alkali metal esters of a ferric C.sub.12-14 -alkylsulfuric acid ester, alkali metal salts of monolauric acid glyceride sulfuric acid ester, and alkali salts of N-octadecylsulfosuccinic acid. By this proposed method, chloroprene rubber having a very light color can be obtained, but the tackiness attributed to an alkali metal salt of a condensate of naphthalenesulfonic acid with formaldehyde is deteriorated to some extent and thus the resulting chloroprene rubber adhesive still has a problem.
In view of the foregoing, there is an increasing demand for developing a chloroprene rubber adhesive having a shade of white only slightly tinged with pale yellow, and good tackiness.